The present invention relates to contact exposure units used in the photographic reproduction of large sized drawings or blueprints, and, more specifically relates to a contact exposure unit adapted to accommodate varying thicknesses of sheet material as well as maintain the original and light sensitive sheets in registry with each other.
Conventional contact exposure units include a housing with a transverse horizontal slot extending from a front or infeed end to a rear or exit end. Upper and lower margins of the slot are provided with a plurality of opposed pairs of fixed upper and lower cylindrical rolls, each roll having a covering of some sort of resilient, rubber-like material. At least one of the rolls is driven so as to pull the sheet materials through the slot. An endless belt is often used to draw the sheets through the unit. A light unit is mounted in the housing to expose sheet materials inserted through the slot for reproduction. For the purposes of the present invention, "sheet materials" refers to the original drawing or blueprint, and the light sensitive reproduction sheet, the latter often being some sort of film. For successful reproductions, the original must be transparent or translucent to allow light to penetrate therethrough.
In operation, the original and the light sensitive sheet are placed in registry with each other and are inserted into the slot with the original closest to the light unit, the original being either face up or face down. As the rolls or the belt pull the sheets through the slot, the light unit is illuminated, which exposes the light sensitive sheet except for areas where masked by lines on the original, creating a latent image. Once exposed, the light sensitive sheet may be transferred to a separate developer unit for further processing.
A drawback of conventional exposure units is that the arrangement and operation of the rolls tends to cause the original and light sensitive sheets to shift relative to each other so that accurate registry is not maintained. This results in unsatisfactory reproductions, especially when the original includes paste-ups or stuck-on portions.
Another drawback of conventional exposure units is that they require complicated static electricity generators, glass panels, or vacuum units to maintain the sheets in a flat condition for accurate exposures.
A further drawback of conventional exposure units is that the fixed separation between the upper and lower rolls renders the unit unable to adequately accommodate originals and/or light sensitive sheets of varying thicknesses.
A still further drawback of conventional exposure units is that the light unit is unable to direct the light to sufficiently illuminate the original to create a sharp latent image on the contact sheet so that line weights in the reproduction are consistent with the original
Thus, there is a need for a contact exposure unit which is adapted to maintain accurate registry between the original and the light sensitive sheet, to accommodate sheet materials of varying thicknesses, and to provide a light unit which creates sharp, and accurate latent images.